The purpose of the injectors in a fuel injection system is to inject the fuel volume requested by the control unit into the combustion chamber of the engine at a predefined point in time. Common rail diesel injection systems having so-called piezo injectors, which have a piezoelectrically driven actuator module, have been in production use for several years. One of the special design features of piezo injectors is the hydraulic coupler. The hydraulic coupler equalizes temperature-related changes in the length of the actuator module. One prerequisite for proper functioning of the injector is that the coupler space be completely filled with fuel during operation. An incompressible liquid volume in the coupler space then allows force to be transferred from the actuator to the switching valve of the injector.
A further prerequisite for proper functioning of the injector is that a counter-pressure be present in the injector return line. The counter-pressure is generated as a rule by a constant pressure valve (CPV) or a throttling element located in the injector return line.
If the coupler space is not completely filled with fuel, and if instead an air cushion has accumulated in the coupler space, this results in negative effects on injector function. Force transfer from the actuator to the switching valve is impaired as a result of the compressibility of the air/fuel mixture in the coupler space. The injection volume decreases as a function of the size of the enclosed air volume; in the worst case the injection volume becomes zero. The result of this is that the engine is no longer capable of running.
The air inclusion can result, for example, from repair work on the fuel system, running the tank dry, defects in the constant pressure valve, fuel degassing effects, or incorrect installation or storage of the injector.
Lack of counter-pressure in the return line, as well as failure to inject because of an empty coupler, result in engine missing and failure to start. Neither defect can be detected by the control unit; this makes fault localization very difficult.
Also known are numerous end-of-the-line faults in which vehicles can no longer be started because of empty couplers or defective constant pressure valves. The subsequent troubleshooting in such cases is always complex.
In order to restore injector function and allow the engine to run again, the only possibility that remains in many cases is to replace emptied injectors with new parts. As delivered, the injectors are filled at the factory with a testing oil in order to ensure injector function.
British Published Patent Appln. No. 2 277 386 describes a diagnostic system for detecting malfunctioning fuel injectors for an internal combustion engine, having an injector controller for individual application of control to the injectors, which are connected to a fuel distributor. Pressure sensors are mounted on the fuel distributor in order to detect pressure waves that result from the operation of the individual injectors.
A signal processing apparatus is provided in order to process the pressure signals of the pressure sensors. The output signal of the signal processing apparatus corresponds to the fuel flow rate through the fuel injectors. An output signal of this kind can be used to inform an operator of the engine of a malfunction of the fuel injectors. The output signal can also be delivered to a fuel injector control apparatus in order to adapt the actuation duration of the injectors so that a desired flow rate for each activation is achieved.